Electron-beam microlithography apparatus are known for use in manufacturing integrated circuits and the like, including such apparatus that "reductively" (i.e., with demagnification) transfer a pattern defined by a mask onto a wafer using a two-stage projection lens. In this regard, reference is made to Japanese Laid-Open patent document no. HEI 5-160012. With such apparatus, it is generally not possible to simultaneously illuminate the entire mask with the electron beam. Consequently, the mask is typically subdivided into multiple mask subfields and the mask pattern is transferred to the wafer one subfield at a time. See, e.g., U.S. Pat. No. 5,260,151.
Electron-beam projection-optical systems used in conventional electron-beam microlithography apparatus include "SMD" (Symmetric Magnetic Doublet) and "PREVAIL" (Projection Exposure by Variable Axis Immersion Lens) systems. Such projection-optical systems are usually "two-stage" by which is meant that the projection-optical system comprises two individual projection lenses arranged in tandem along an optical axis.
It is desired that a two-stage demagnifying projection-optical system based on the SMD concept allow adjustment of the focal point of the projection-optical system without changing the magnification of the projection-optical system and without causing rotation of the image produced by the projection-optical system. Adjustments to the focal point can be made by increasing or reducing, by the same magnitude, the electrical current applied to the constituent projection lenses.
In certain conventional projection-optical systems, one projection lens has a "split" coil that, in the direction of the optical axis, comprises two lens coils. A split coil allows alteration of a ratio of electrical current flowing through the pair of coils while maintaining the same total current applied to the respective projection lens. Reference is made to Lischke et al., "Investigations about High Performance Electron-Microprojection Systems," Proceedings of the Eighth Int. Conf. on Electron and Ion Beams in Science and Technology, Seattle (USA), 1978.
Japanese Laid-Open patent document no. HEI 7-235469 discloses an electron-beam projection-optical system consisting of a first projection lens that primarily adjusts the magnification of the transferred image and a second projection lens that primarily adjusts rotation of the image.
The conventional projection-optical systems summarized above do not adequately control aberrations generated whenever the lens magnification is changed or whenever the image rotation is adjusted. These projection-optical systems also do not adequately minimize changes in image rotation and/or focal point that arise when the magnification is changed.